An automatic transmission mountable on a vehicle is equipped with a plurality of frictional engagement elements such as a clutch element and a brake element, and configured to selectively engage one or more of the frictional engagement elements depending on an engine operating state to thereby change a driving force transmission path of a speed change mechanism comprising planetary gear sets, to automatically perform a speed change operation to establish a given speed stage.
A brake element described in the following Patent Literature 1, comprises a first piston configured to press friction plates, and a second piston configured to press the first piston toward a side close to the friction plates, wherein the first piston and the second piston are arranged in series along a stroke direction thereof, in this order from the side of the friction plates. The brake element further comprises a return spring acting on the first piston to bias the first and second pistons toward a side away from the friction plates. When the brake element is in a disengaged state, both of the first and second pistons are moved toward the side away from the friction plates, and set at their respective initial positions.
Thus, when a possibility arises that the brake element is engaged, the second piston can be preliminarily moved by a stroke toward the side close to the friction plates to thereby move the first piston by a stroke toward the side close to the friction plates, so that it becomes possible to preliminarily narrow a clearance in the friction plates (The resulting position of the first piston and the resulting position of the second piston will hereinafter be referred to respectively as “standby position of the first piston” and “standby position of the second piston”).
Then, when it becomes necessary to engage the brake element, the first piston can be moved from the standby position by a stroke toward the side close to the friction plates, so that it becomes possible to establish effective pressing of the friction plates by the first piston within a short period of time, because the clearance is preliminarily narrowed (The resulting position of the first piston will hereinafter be referred to as “pressing established position”). This makes it possible to engage the brake element with good responsiveness, i.e., engage the brake element accurately at an adequate timing, to thereby suppress a speed-change shock or the like due to a deviation in engagement timing of the brake element.
Meanwhile, in the brake element described in the Patent Literature 1,, the return spring acts on the first piston. Thus, every time the brake element is disengaged, each of the first and second pistons is returned to a position most away from the friction plates, in a stroke range thereof. More specifically, the initial position of the first piston is a position most away from the friction plates, in a stroke range thereof, and the initial position of the second piston is also a position most away from the friction plates, in a stroke range thereof, so that a relative positional relationship between the first and second pistons at their initial positions never changes. Further, the second piston is moved by a stroke toward the side close to the friction plates, until this movement is restricted by a wall surface of a transmission casing, a stopper member or the like, so that the standby position of the second piston is structurally determined That is, the standby position of the first piston whose initial position relative to the initial position of the second piston never changes is also structurally determined, so that the standby position of the first piston never changes.
Thus, for example, in a situation where the friction plates have a relatively large clearance due to an individual difference between brake elements, or in a situation where the friction plates are worn away due to aging to cause an increase in the clearance, a distance between the standby position and the pressing established position of the first piston increases. This results in an increase in time necessary for the first piston to establish effective pressing of the friction plates, causing deterioration in responsiveness regarding engagement of the brake element and thus deterioration in accuracy of engagement timing of the brake element.